20-10-2016, 02:28 PM
1460181490-v2vBrandonWilson.ppt (Size: 947.5 KB / Downloads: 20)
Motivation
There were nearly 6,420,000 auto accidents in the United States in 2005
The financial cost of these crashes was more than 230 billion dollars
6858 people were injured in road accidents in 2006
60% of collisions could be avoided given at least 0.5 sec warning
Cooperative Collision Warning
Scenario: A, B, and C traveling in same direction. A suddenly brakes.
Using V2V the danger for all parties is alleviated:
A can send warning messages immediately once emergency is detected
Assuming little delay, B and C can receive the alerts and react
Challenges
1) Stringent delay requirement (in the order of ms)
A vehicle traveling 80mph covers > 1m per second
Several obstacles:
Doppler shifts from high mobility
Packet collision rate
Large churn in the pool of endangered vehicles
2) Support for multiple co-existing vehicles over long period
Emergencies may take hours to clear
By nature, road emergencies have chain reaction
3) Differentiation of emergency events and redundant messages
Vehicle trajectory helps differentiate emergency events
Redundant messages can overload the communication channel
Vehicle Collision Warning Communication (VCWC) Protocol
Definitions and abbreviations:
Abnormal Vehicle (AV)
Emergency Warning Message (EWM)
Geographical location
Speed
Acceleration
Moving direction
Assumptions about participating vehicles:
Capable of determining geographic location relative to road (GPS)
Must be equipped with at least one wireless transceiver
Transmission range is assumed to be 300m (Dedicated Short Range Communications (DSRC) consortium suggestion)
Congestion Control
Goal: achieve low EWM delivery delay at time of emergency while scaling to many co-existing AV’s
EWM delivery delay from A to V – elapsed duration from time of emergency event at A until message received by V
Delay = Delaywait + Delaytransmission
Congestion traditionally regulated via transmission success rate
Will not work in multicast scenario
VCWC uses multiplicative transmission rate decreasing
Transmission rate is decreased by a factor of a after every L transmissions
a = 2 and L = 5 determined empirically
State Transitions of AVs
Goal: Ensure maximal message dissemination while eliminating redundant messages clogging the network
AV can be in three states:
Initial AV – transmits at initial rate, λ0, and decreases using multiplicative decrease
Flagger AV – transmits at minimal rate λmin
Non-flagger AV – does not transmit
Transition from initial AV to non-flagger if both:
At least Talert time passed since entering initial AV state
EWM’s from at least one follower is overheard
X is follower of Y if X located in lane behind Y and all vehicles endangered by Y are endangered by X